The present invention is directed to the formation of color proofs. In general, the image in a color proof is formed by transferring a colorant (e.g., dye or pigment) from a donor to a receptor under the influence of energy from a thermal printhead or a laser. This transfer can occur via mass transfer or dye transfer.
In a mass transfer system, the majority of the material on the donor (e.g., colorant, binder, and additives) is transferred to the receptor. Typically, this can occur either by a melt mechanism or by an ablation (i.e., ablative) mechanism. In a melt mechanism, the donor material is softened or melted. This softened or molten material then flows across to the receptor. This is typically the mechanism at work in a conventional, thermally induced wax transfer system. In an ablation mechanism, gases are typically generated that explosively propel the donor material across to the receptor. This results from at least partially volatilizing the binder or other additives in and/or under a layer of the donor material to generate propulsive forces to propel the colorant toward the receptor.
In a dye transfer system, however, only the colorant is transferred from the donor to the receptor. That is, the colorant is transferred without binder or other additives. This can occur either by a diffusion mechanism or a sublimation mechanism.
The image in a color proof formed from a mass transfer system is typically a half tone image. In a system that forms half tone images, the transfer gives a bi-level image in which either zero or a predetermined density level is transferred in the form of discrete dots (i.e., pixels). These dots can be randomly or regularly spaced per unit area, but are normally too small to be resolved by the naked eye. Thus, the perceived optical density in a half tone image is controlled by the size and the number of discrete dots per unit area. The smaller the fraction of a unit area covered by the dots, the less dense the image will appear to an observer.
The image in a color proof formed from a thermal dye transfer system is typically a continuous tone (i.e., contone) image. In a continuous tone or contone image, the perceived optical density is a function of the quantity of colorant per pixel, higher densities being obtained by transferring greater amounts of colorant.
To emulate half tone images using a thermal dye transfer system, which typically forms a contone image, the laser beam can be modulated by electronic signals which are representative of the shape and color of the original image, so that each dye is heated to cause volatilization only in those areas in which its presence is required on the receptor to reconstruct the color of the original object. Further details of this process are disclosed in GB Publication No. 2,083,726 (3M Company). U.S. Pat. Nos. 4,876,235 (DeBoer) and 5,017,547 (De Boer) also disclose a thermal dye transfer system in which the perceived optical density is obtained by controlling the tonal gradation or thickness (density) of the colorant per pixel. In this system, the receptor includes spacer beads to prevent contact between the donor and receptor elements. This allows for the dye to diffuse or sublime across to the receptor element without the binder.
The shape and/or definition of the dots can effect the quality of the image. For example, dots with more well-defined and sharper edges will provide images with more reproducible and accurate colors. The shape and/or definition of the dots are typically controlled by the mechanism of transfer of the image from the donor to the receptor. For example, as a result of the propulsive forces in an ablative system, there is a tendency for the colorant to "scatter" and produce less well-defined dots made of many fragments. Attempts have been made to produce more well-defined dots using an ablative system described in U.S. Pat. Nos. 5,156,938 (Foley) and 5,171,650 (Ellis); however, whether single layer or dual layer, such systems do not produce contract-quality images. In contrast, systems involving the transfer of molten or softened material can in principle form more well-defined dots.
For imaging by means of laser-induced transfer, the donor element typically includes a support bearing, in one or more coated layers, an absorber for the laser radiation, a transferrable colorant (e.g., one or more dyes or pigments), and one or more binder materials. When the donor element is placed in contact with a suitable receptor and subjected to a pattern of laser irradiation, absorption of the laser radiation causes rapid build-up of heat within the donor element, sufficient to cause transfer of colorant to the receptor in irradiated areas. By repeating the transfer process using different donor elements and the same receptor, it is possible to superimpose several monochrome images on a common receptor, thereby generating a full color image. This process is ideally suited to the output of digitally stored image information. It has the additional benefits of not requiring chemical processing and of not employing materials that are sensitive to normal white light.
As discussed above, laser-induced transfer can involve either mass transfer of the binder, colorants, and infrared absorber, giving a bi-level image in which either zero or maximum density is transferred (depending on whether the applied energy exceeds a given threshold), or dye sublimation transfer, giving a continuous tone image (in which the density of the transferred image varies over a significant range with the energy absorbed). Laser-induced mass transfer has been characterized in the literature, in Applied Optics, 9, 2260-2265 (1970), for example, as occurring via two different modes. One mode involves a less energetic mode in which transfer occurs in a fluid state (i.e., by melt transfer), and one mode involves a more energetic mode in which transfer occurs by an explosive force, as a result of generation and rapid expansion of gases at the substrate-coating interface (i.e., by ablation transfer). This distinction has also been recognized in U.S. Pat. Nos. 5,156,938 (Foley), 5,171,650 (Ellis), 5,516,622 (Savini), and 5,518,861 (Covalaskie), which refer to ablation transfer as a process distinct from melt transfer, and refer to its explosive nature, as opposed to U.S. Pat. Nos. 5,501,937 (Matsumoto), 5,401,606 (Reardon), 5,019,549 (Kellogg), and 5,580,693 (Nakajima), which refer to transfer of a colorant in a molten or semi-molten (softened) state, with no mention of explosive mechanisms.
Many of the known laser-induced melt transfer systems employ one or more waxes as binder materials, in pursuit of a transfer layer that melts sharply to a highly fluid state at moderately elevated temperatures, and hence gives a higher sensitivity; however, such systems are prone to image spread as a result of wicking or uncontrolled flow of the molten transfer material. Furthermore, because the laser absorber is normally transferred along with the desired colorant, the final image may lack the accuracy of color rendition required for high quality proofing purposes. Others have attempted to increase the sensitivity by adding plasticizers (see, e.g., U.S. Pat. No. 5,401,606 (Reardon)), which lowers the melt viscosity and increases the flow; however, such additives soften the films such that they become receptive to impressions and blocking, for example.
Thus, there is still a need for a laser-induced thermal transfer system that provides a half tone image in the form of discrete dots having well-defined, generally continuous edges that are relatively sharp with respect to density or edge definition (i.e., not feathered).